Weldable,nonmagnetic austenitic manganese steel

ABSTRACT

A WELDABLE, NONMAGNETIC AUSTENITIC MANGANESE STEEL OF THE HADFIELD TYPE AND COMPATIBLE WELD METAL WHICH WILL REMAIN NONMAGNETIC AND TOUGH AS WELDED AND WITHOUT THE REQUIREMENT OF POST-WELD HEAT TREATMENT COMPRISING .3% CARBON, 13% MANGANESE, 5% NICKEL, 3 TO 5% CHROMIUM, 1% MOLYBDENUM AND .5% VANADIUM.

April 13, 1971 TRUE STRESS, I000 psi ENGINEERING STRESS, IOOOpsi A. M.HALL ETAL WELDABLE, NONMAGNETIC AUSTENITIC MANGANESE STEEL Filed June24, 1968 2 Sheets-Sheet 1 I25 X-'-)" X X -X"X/ I00 STRAIN, INCH PER INCHX X/ I50 X/ x/ x I00 x TRUE STRAIN INVENTORS ALBERTM. HALL F/ 2 D/MO/VA. ROBE/7 rs 6. DONALD B.R0A0H FRANK 4. P/MEN TEL v WELDABLE,NONMAGNETIC AUSTENITIC MANGANESE STEEL Filed June 24, 1968 71 A. M.HALL. ETAL ApriI 13, E

2 Sheets-Sheet 2 X M. X

STRAIN, INCH PER INCH E509 wmmmtw MS;

TRUE STRAIN INVENTORS ALBERTM. HALL 00mm 9. mac/1 D/MON 4.9055 i' g F/64 FRA/wmP/MEir Wm B N R m T A United States Patent C Filed June 24,1968, Ser. No. 739,305 Int. Cl. C22c 39/20 U.S. Cl. 75-428 3 ClaimsABSTRACT OF THE DISCLGSURE A weldable, nonmagnetic austenitic manganesesteel of the Hadfield type and compatible weld metal which will remainnonmagnetic and tough as welded and without the requirement of post-weldheat treatment comprising 3% carbon, 13% manganese, 5% nickel, 3 to 5%chromium, 1% molybdenum and .5 vanadium.

BACKGROUND OF THE INVENTION The present invention relates to manganesesteel of the Hadfield type and more particularly to a weldable,nonmagnetic austenitic manganese steel and compatible weld metal toreplace Hadfields manganese steel in certain marine applications.

Austenitic manganese steel of the Hadfield type, containing about 12 to14% manganese and 1.2% carbon, is used in a number of important navalapplications. These steels are employed as cast and as wrought productsand as a general rule are placed in service in the annealed conditionwhich is achieved by water quenching from 1850 F. and above. In thiscondition they are nonmagnetic and exhibit excellent toughness and thecapability to resist battering and abrasion resulting from heavyimpacting.

The austenitic manganese steels have found wide usage in manyapplications characterized by heavy impacting, wear, and abrasion in thehandling of bulk materials, e.g. balls for grinding mills. Anotherapplication is for helmets and body armor for military personnel. Stillother applications of the austenitic manganese steels hinge on theirnonmagnetic quality. In this category are parts for minesweepers, partsfor lifting magnets, and special electrical equipment.

Hadfields steel, however, does have certain deficiencies which detractfrom its usefulness and service performance. The steel must be properlyannealed (Water quenched from about 1900 F.) to exhibit high-toughnesscharacteristics; plate or bar stock slow cooled from the annealingtemperature will exhibit significantly reduced impact properties (CharpyV-notch valves as low as 20 ft.-lb.). The reduction in impact strengthcan be attributed to continuous grain-boundary carbide precipitationoccurring in the temperature range from about 1600 to 600 or 700 F.Grain-boundary carbide precipitation is also equally detrimental inwelding; Weld-heat-afiected zones frequently exhibit Charpy V-notchimpact values in the order of ft.-lb. or less.

The grain-boundary carbide precipitation resulting from exposure tomoderate temperatures can be attributed directly to the high carboncontent of the steel, however, reductions in the carbon content reducethe amount and severity of the intergranular carbide precipitation thatmay occur. However, drastic reductions in carbon content (to perhaps0.10% or less) are required to eliminate the thermally inducedembrittlement in weld-heat-affected zones and in slow-cooled plate andbar. Because carbon is a potent strengthener and austenite stabilizer inthese high-manganese austenitic steels, reductions in carbon contentsuflicient to result in acceptable resistance to thermally inducedembrittlement in weldments are accompanied by a magnetic response in thesteel at room temperatures. In reducing the carbon content of the steel,the temperature at which martensite starts to form on cooling from hightemperatures (the M temperature) increases from cryogenic temperaturesto temperatures approaching room temperature. At low carbon contents,the 12 to 14 percent manganese steel may have an M temperatureapproaching or above room temperature. The presence of martensite, ofcourse, produces a magnetic response in the steel. In addition,martensite formation also reduces the toughness of the steel.

Thus, while Hadfields steel is an excellent material for applicationsrequiring completely nonmagnetic characteristics and a high level oftoughness, the steel does not retain these characteristics when welded.Weld deposits and weld-heat-afiected zones exhibit toughness propertiesconsiderably inferior to those of annealed material. Efforts to improvethe toughness of Welded joints through the addition of alloying elementssuch as nickel, chromium and molybdenum have not been successful.However, it has been found that by adding nickel and reducing the carboncontent of Hadfields manganese steel, alloys more resistant to thermallyinduced embrittlement can be obtained. For example, welding electrodescontaining about 12 percent manganese, 2.75 to 5.0 percent nickel and0.50 to 0.90 percent carbon are commercially available and can beemployed to produce multipass Weld deposits with improved toughness. Ofcourse, the electrode employed to join Hadfields steel does not affectthe development of brittleness in parent metal heat-efiected zones.

The best prior art means to insure the characteristic toughness ofaustenitic manganese steel is to employ a postweld annealing treatmentat 1850 F. or above, followed by Water quenching. Such postweld heattreatment is troublesome, impractical and often not feasible.

SUMMARY The general purpose of this invention is to provide an alloythat has all the advantages of Hadfields steel and none of theabove-described disadvantages. The alloys of this invention possessingthe properties indicated above, fall within the following compositionranges, in percent,

Iron-Balance plus incidental impurities.

Typical properties of the alloy of this invention, in the annealedcondition, are as follows:

Yield strength45,000 p.s.i.

Tensile strength-115,000 p.s.i. Elongation% Impact strength(annealed)l55 ft.-lb.

Impact strength (sensitized at 1200 F. for 1 hour)- ft.-1b.

Accordingly, it is an object of the invention to provide a nonmagneticsteel of the austenitic manganese type.

A further object of the invention is to provide a non magnetic steel ofthe austenitic manganese type which can be welded by standardarc-welding processes without developing weld-heat-affected zones.

A still further object of the invention is to provide a steel which isresistant to general attack in seawater and resistant tostress-corrosion cracking in seawater when employed at yield-10astresses.

Another object of the invention is to provide a compatible weld metal.

Still another object is to provide a compatible weld metal that willremain nonmagnetic and tough as welded and without the requirement ofpostweld heat treatment.

BRIEF DESCRIPTION OF THE DRAWINGS The exact nature of this invention aswell as other objects and advantages thereof will be readily apparentfrom consideration of the following specification relating to theannexed drawing in which:

FIGS. 1 and 2 are graphical representations of Engineering Stress-Strainand True Stress-Strain curves for 0.505 inch diameter tensile specimensmachined from annealed bar stock of an alloy of this invention.

FIGS. 3 and 4 are graphical representations of Engineering StressStrainand True Stress-Strain curves for All-Weld-Metal Tensile specimenmachined from MIG Weld deposit of an alloy of this invention.

DESCRIPTION OF THE PREFFERED EMBODIMENT The invention is illustrated,but not limited, by the following specific examples of the preparationof a nonmagnetic steel and a compatible weld metal. Wherever possible,alternate modes of preparation are discussed but it will be recognizedthat various additional modifications can be made without deviating fromthe scope of the invention.

The excellent mechanical and physical properties of the alloys of thisinvention are obtained by the careful balance of the alloying elements(carbon, nickel, vanadium and chromium) to maintain nonmagneticstructure, the desired degree of thermal stability, and the desiredstrength properties. Vanadium should be restricted to a maximum of 0.5%.Chromium in amounts up to 5% is advantageous. Carbon should berestricted to the range from 0.25% to 0.40%. At lower carbon content, amagnetic response will be noted in deformed material, and at highercarbon contents, thermal stability is lacking. Nickel is required tomaintain the austenitic structure at the carbon content specified.Increasing nickel above about 6% reduces strength, whereas less than3.5% nickel results in an unstable austenitic structure. Molybdenum haslittle definite effect, however, a 1.0% addition appears to strengthenand maintain an austenitic structure.

While the composition ranges specified above will provide alloys havingthe above mentioned properties, a typical alloy embodying this inventionis as follows:

Percent Carbon 0.30 Manganese 13.0 Nickel 5.0 Vanadium 0.5 Chromium 5.0Molybdenum 1.0

To prepare suflicient material for subsequent welding tests, a 500 poundheat of the above composition was melted, fabricated into bar and plate,and evaluated in mechanical tests. A charge consisting of Armco iron andelectrolytic nickel was melted in a magnesia lined induction furnace.After meltdown, ferromolybdenum, ferrovanadium, and ferrochromium wereadded, followed by the required ferromanganese. The melt was thendeoxidized with ferrosilicon and poured into 100-pound ingot molds. Theanalysis of drillings taken from the ingot hot top was as follows:

The ingots were heated to 1950 F. and forged into 3-inch square bars forrolling into rounds, and into 3- by S-inch slabs for rolling into plate.The forged bars were hot rolled to 1 /8 inch diameter rounds, while theflats were hot rolled into /2 and inch thick plate. The rounds and platewere then annealed at 1900 F. and water quenched.

Specimens for mechanical and magnetic permeability tests were machinedfrom the 1% inch diameter round material. Tensile tests were conductedon duplicate 0.505 inch diameter specimens, employing a strain rate of0.005 inch per minute up to the yield load and a head speed of 0.10 inchper minute thereafter. By measuring the load after each 0.10 inchincrement in elongation, a complete load-deformation curve was obtained.True-stress truestrain curves were constructed for each tensile test.Engineering Stress-Strain and True-Stress True-Strain curves for thismaterial are shown in FIGS. 1 and 2. The results of the tensile testswere as follows:

Earl Bar2 Specimens machined from annealed rounds had impact strengthsof 151, 157 and 156 ft.-1bs. Magnetic-permeability tests indicated thatthe material was nonmagnetic (permeability of 1.003 or less). Moreover,hand magnet tests revealed no evidence of a magnetic response in thefractured areas of the tensile or impact bars.

In preparing base metal and electrode wire for subsequent welding tests,a sample alloy was taken from the 500 pound heat mentioned above. Platematerial /2 inch and inch thick, from this heat, were annealed at 1900F. and water quenched. The plate was grit blasted and cleaned in acetoneprior to welding tests.

In producing filler wire, a quantity of 1% inch round bar stock was hotrolled at 1900 F. to inch diameter rod, which was then centerless groundto remove any surface defects and subsequently annealed at 1900 F. Therod was then cold drawn into 0.035 inch diameter wire, cleaned, andspooled for welding tests. The rod work-hardened rapidly during drawingoperations. In gages above 0.090 inch, it was necessary to anneal therod after every 30 percent cold reduction. In sizes smaller than 0.090inch, the wire could be cold reduced 50 percent between annealprocesses. Annealing was performed at 1900 F. in an argon atmosphereretort and cooled in the retort. After final annealing, the wire wasdrawn through one die for straightening purposes, cleaned and spooled.

All gas metal-arc (MIG) welds were made in the flat position employingautomatic welding equipment and a 0.035 inch diameter electrode producedfrom an alloy of this invention. Single-V butt joints were made in /2and inch plate of this alloy. The joints were evaluated by means of thefollowing tests:

(1) Transverse weld-metal tension tests (2) All weld-metal tension tests(3) Transverse-weld side bend tests (4) Transverse-weld Charpy V-notchimpact tests Hatch-Hartbower type, weld-metal heat-affected-zone impacttests (6) Battelle variable restraint cracking tests.

Tensile tests-Transverse weld specimens The results of tensile tests onthe round (0.505 inch) and rectangular /2 x A? inch) specimens were asfollows:

1 Specimens broke in the grips where the extension tabs had been welded.

All 0.505-inch-diameter specimens failed in the weld deposit indicatingthat the heat-aifected zones were stronger than the weld deposit. Theyield strengths of these transverse weld specimens were significantlyhigher (about 8,000 to 10,000 p.s.i.) than that of annealed platematerial of this alloy. The tensile strengths of the weld specimens wereslightly below that of the annealed wrought material. However, thetransverse weld specimens exhibited less than half the ductility of theannealed wrought material. Nevertheless, a tensile elongation of 25percent is considered acceptable.

Tensile tests-All-weld-metal specimens The results of tensile tests wereas follows:

Ultimate 0.2% ofiset yield tensile Elongation Bar strength, p.s.i.strength, p.s.i. percent in 2 in.

Side-bend tests Transverse-weld side-bend tests were conducted onspecimens 6 inches long by inch wide by inch thick, machined from weldsin inch plate so that the width of the specimen represented the completeWeld thickness. The results showed that all three specimens could bebent around a die having a radius of inch without cracking in theheat-affected zone or at the fusion line. Some slight tearing of theweld metal was noted in bending over the inch radius. This tearing wasmanifested by small fissures less than A inch long near the center ofthe weld.

Impact tests The results of Charpy V-notch impact tests made intransverse weld specimens and on Hatch-Hartbower type specimens were asfollows:

Specimen type: Impact valve, ft.-lb. Transverse weld 67, 54, 52Hatch-Hartbower 125, 123, 123, 131, 123

In addition, measurements were made of the expansion occurring beneaththe notch in the Hatch-Hartbower specimens. This expansion is indicativeof the ductility of the material in the presence of a V-notch and underimpacttest loading conditions. These measurements showed an averageexpansion of 0.0276 inch for the weld-metal portion and 0.0303 inch forthe heat-aifected-zone portion of the specimens. In comparison, similarmeasurements made on broken impact specimens of annealed material ofthis alloy revealed an average expansion of 0.0357 inch. (This annealedmaterial had shown an impact strength of about 150 ft.-lb.)

These results indicate that, in the welded specimens, the heat-affectedzones had higher impact strengths than did the weld deposit and that theannealed wrought material had a higher impact strength than did theheatatfected zone and the weld deposit.

Variable-restraint weld tests Tests indicated that weld deposits andheat-affected zones of welded joints in the alloy of this invention arenot susceptible to weld-metal or heat-atfected-zone cracking when underconditions of high restraint. No cracking or fissuring was observed evenin areas of maximum restraint.

Metallographic examination Sections of welded joints made in inch platewere prepared for metallographic examination. Careful examinationrevealed fine discontinuous grain-boundary carbides in theweld-heat-affected zones. At relatively low power (250X), thegrain-boundary carbides could not be clearly resolved in the areasadjacent to the fusion line. At very high power (1500X the finediscontinous nature of the carbide precipitation could be observed.These carbides are not considered embrittling-the heatalfected zone hadshown high impact strength.

Magnetic characteristics Qualitative tests with a strong hand magnet didnot reveal any evidence of a magnetic response in any of the weldedjoints. Furthermore, failed tensile and impact specimens also did notexhibit evidence of magnetic response in heavily deformed areas adjacentto the fractures.

From the foregoing it is apparent that the alloys of this inventionprovide a nonmagnetic steel of the austenitic manganese type which canbe welded by standard arc-welding processes without developingweld-heat-affected zones and which are resistant to general attack inseawater and resistant to stress-corrosion cracking in seawater whenemployed at yield-load stresses.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A weldable, nonmagnetic, austenitic manganese steel consistingessentially of:

Iron-Balance plus incidental impurities.

2. A weldable, nonmagnetic, austenitic manganese steel consistingessentially of:

Percent Carbon 0.20-0.30 Manganese 12.0- Nickel 4.0-6.0 Chromium 3.0-5.0Molybdenum 0.75-1.50 Vanadium 0.20-0.50

IronBalance plus incidental impurities.

3. A weldable, nonmagnetic, austenitic manganese steel consistingessentially of:

Percent Carbon 0.30 Manganese 13.0 Nickel 5.0 Vanadium 0.5 Chromium 5.Molybdenum 1.0

with the balance iron.

References Cited UNITED STATES PATENTS 2,026,468 12/1935 Hall 75128A2,156,299 5/1939 Leitner 75128A 5 2,865,740 12/1958 Heger 75-128 HYLANDBIZOT, Primary Examiner US. Cl. X.'R. 7s 12s

